1. Field of the Invention
The present invention relates to a balanced SAW filter having a balanced-to-unbalanced transforming function, and in particular, the present invention relates to a balanced SAW filter including five IDT type first and second longitudinally-coupled-resonator SAW filter elements that are connected in a cascade arrangement.
2. Description of the Related Art
Surface acoustic wave (SAW) filters are widely used as band-pass filters in mobile communication devices, such as cellular phones. In particular, as cellular phones and other mobile communication devises become smaller and have higher performance, so-called balanced SAW filters with a balanced-to-unbalanced transforming function are used as RF band-pass filters connected to antenna terminals. An exemplary balanced SAW filter of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2000-91883 (Patent Document 1), which will be described below.
As shown in FIG. 3, a balanced SAW filter 100 described in Patent Document 1 includes first and second longitudinally-coupled-resonator SAW filter elements 101 and 102, which are connected in a cascade arrangement. The first longitudinally-coupled-resonator SAW filter element 101 includes first to fifth IDTs 111 to 115 arranged in a propagation direction of surface acoustic waves. Reflectors 116 and 117 are disposed on either side of an area in which the IDTs 111 to 115 are disposed.
Similarly, the second longitudinally-coupled-resonator SAW filter element 102 includes sixth to tenth IDTs 121 to 125 arranged in the propagation direction of surface acoustic waves, and reflectors 126 and 127.
One end of each of the second and fourth IDTs 112 and 114 is commonly connected to an unbalanced terminal 131. One end of each of the first, third, and fifth IDTs is commonly connected. The other end of the first IDT 111 is connected via a first wiring line 141 to the sixth IDT 121. Similarly, the third and fifth IDTs 113 and 115 are connected via second and third wiring lines 142 and 143 to the eighth and tenth IDTs 123 and 125, respectively. In contrast, one end of the seventh IDT 122 is connected to a first balanced terminal 132, and the other end of the IDT 122 is commonly connected to one end of the IDT 124. The other end of the IDT 124 is connected to a second balanced terminal 133. Since the SAW filter 100 described in Patent Document 1 has the above-described structure, the SAW filter 100 can be miniaturized and can achieve lower insertion loss.
Although the balanced SAW filter 100 includes two cascaded five IDTs type longitudinally-coupled-resonator SAW filter elements 101 and 102, signals having the same phase flow through the first to third wiring lines 141 to 143.
In contrast, a wiring line 144 electrically connecting the second balanced terminal 133 to the ninth IDT 124 is, as shown in FIG. 3, extracted between the second and third wiring lines 142 and 143 beyond a portion in which the first and second wiring lines 141 and 142 are disposed. A wiring line 145 connecting the first balanced terminal 132 to the IDT 122 is disposed in an area separated from an area in which the first to third wiring lines 141 to 143 are disposed.
Therefore, direct waves based on stray capacitance between the first to third wiring lines 141 to 143 and the first and second balanced terminals 132 and 133 are strong, and the strong direct waves are applied to the balanced terminals 132 and 133. The strength of the direct waves applied to the balanced terminal 133 is considerably greater than the strength of the direct wave applied to the balanced terminal 132. As a result, the balance between the balanced terminals 132 and 133 is poor.